(1) Regulatory Analysis a) Examination of cell substrates used for vaccine production for the presence of adventitious agents. b) Identification of extraneous material in vaccine samples. (2) Research a) Studies on the effect of MHC Class I genes in the regulation of retroviral production in mouse melanoma cells. Using immunoelectron microscopy we have shown that transfection of the H-2Kb or H-2 Kd, but not the H-2Dd or H-2Ld genes, results in the virtual elimination of two actively produced endogenous retroviruses. The absence of the intracisternal A-type particles was found to be due to lack of IAP-specific mRNA, while the loss of C-type retroviral particles was associated with rearrangements of the proviral DNA. b) The biochemical mechanisms that switch melanocytes between the production of eumelanins and pheomelanins involve the opposing action of a-Melanocyte Stimulating Hormone and Agouti Signal Protein (ASP). Melanosomes in black murine melanocytes are of two basic types, football-shaped and round. Following exposure of these cells to purified recombinant ASP, cell pigmentation decreased. Electron microscopy showed that the oval melanosomes had decreased significantly in number and melanin content while the round type remained unchanged. Chemical analysis confirmed a 90% decrease in eumelanin content with no change in the pheomelanin content. (3) Research support The EM Staff has provided collaborative support for a wide variety of research projects initiated by CBER scientists: a) Immunogold labeling of surface antigens on tuberculosis, pertussis and streptococcus bacteria. b) Immunogold labeling of Respiratory Syncytial Virus fusion protein in infected Hep-2 cells. c) Screening cultures for evidence of viral replication. d) Examination of liver biopsies of chimpanzees infected with Hepatitis C. e) Identification of apoptotic cell death in Lymphokine Activated Killer cells. f) Determination of the content and homogeneity of subcellular fractions.